This story is probably my personal favorite conspiracy story. Late in the 19th century, the priest of a small village in Southern France came across a small piece of paper while renovating his church. This led him to a discovery that turned him insanely rich, practically overnight. Investigations into the source of his wealth have lead investigators to the Templar Knights, the Holy Grail, Jesus and Mary Magdalene, strange anomalies in the surrounding landscape, … and eventually ignited the The Da Vinci Code craze. If you dig even deeper, you’ll even get to the mystery of the theft of the painting Adoration of the Mystic Lamb in Ghent (Belgium). That, however, is a story for a different time.
Timewatch - The History of a Mystery (1996)